1. Field
This application relates to masking fabrication, such as using masks in the fabrication of solar cells.
2. Related Art
During various fabrication stages of solar cells, it may be desired to use masks so as to block parts of the solar cells from the particular fabrication process. For example, masks may be used for formation of contacts or for edge exclusion to preventing shunting of the cell. That is, for cells having contacts on the front and back sides, materials used for making the contacts may be deposited on the edges of the wafer and shunt the front and back contacts. Therefore, it is advisable to use mask to exclude the edges of the cell during fabrication of at least the front or back contacts.
As another illustration, for the fabrication of silicon solar cells, it is desirable to deposit blanket metals on the back surface to act as light reflectors and electrical conductors. The metal is typically aluminum, but the blanket metals could be any metal used for multiple reasons, such as cost, conductivity, solderability, etc. The deposited film thickness may be very thin, e.g., about 10 nm up to very thick, e.g., 2-3 um. However, it is necessary to prevent the blanket metal from wrapping around the edge of the silicon wafer, as this will create a resistive connection between the front and back surfaces of the solar cell, i.e., shunting. To prevent this connection, an exclusion zone on the backside edge of the wafer can be created. The typical dimension of the exclusion zone is less than 2 mm wide, but it is preferable to make the exclusion as thin as possible.
One way to create this exclusion zone is through the use of a mask; however, using masks has many challenges. Due to the highly competitive nature of the solar industry, the mask must be very cheap to manufacture. Also, due to the high throughputs of solar fabrication equipment (typically 1500-2500 cells per hour), the mask must be quick and easy to use in high volume manufacturing. Also, since the mask is used to prevent film deposition on certain parts of the wafer, it must be able to absorb and accommodate deposition build up. Furthermore, since film deposition is done at elevated temperatures, the mask must be able to function properly at elevated temperature, e.g., up to 350° C., while still accurately maintaining the exclusion zone width, while accommodating substrate warpage due to thermal stresses.